Noise control constitutes a rapidly growing economic and public policy concern for the construction industry. Areas with high acoustical isolation (commonly referred to as ‘soundproofed’) are requested and required for a variety of purposes. Apartments, condominiums, hotels, schools and hospitals all require rooms with walls, ceilings and floors that reduce the transmission of sound thereby minimizing, or eliminating, the disturbance to people in adjacent rooms. Soundproofing is particularly important in buildings adjacent to public transportation, such as highways, airports and railroad lines. Additionally theaters, home theaters, music practice rooms, recording studios and the like require increased noise abatement. Likewise, hospitals and general healthcare facilities have begun to recognize acoustical comfort as an important part of a patient's recovery time. One measure of the severity of multi-party residential and commercial noise control issues is the widespread emergence of model building codes and design guidelines that specify minimum Sound Transmission Class (STC) ratings for specific wall structures within a building. Another measure is the broad emergence of litigation between homeowners and builders over the issue of unacceptable noise levels. To the detriment of the U.S. economy, both problems have resulted in major builders refusing to build homes, condos and apartments in certain municipalities; and in widespread cancellation of liability insurance for builders. The International Code Council has established that the minimum sound isolation between multiple tenant dwellings or between dwellings and corridors is a lab certified STC 50. Regional codes or builder specifications for these walls are often STC 60 or more. It is obvious that the problem is compounded when a single wall or structure is value engineered to minimize the material and labor involved during construction.
It is helpful to understand how STC is calculated in order to improve the performance of building partitions. STC is a single-number rating that acts as a weighted average of the noise attenuation (also termed transmission loss) of a partition across many acoustical frequencies. The STC is derived by fitting a reference rating curve to the sound transmission loss (TL) values measured for the 16 contiguous one-third octave frequency bands with nominal mid-band frequencies of 125 Hertz (Hz) to 4000 Hertz inclusive, by a standard method. The reference rating curve is fitted to the 16 measured TL values such that the sum of deficiencies (TL values less than the reference rating curve), does not exceed 32 decibels, and no single deficiency is greater than 8 decibels. The STC value is the numerical value of the reference contour at 500 Hz. For maximum STC rating, it is desirable for the performance of a partition to match the shape of the reference curve and minimize the total number of deficiencies.
An example of materials poorly designed for performance according to an STC-based evaluation is evident in the case of many typical wood framed wall assemblies. A single stud wall assembly with a single layer of type X gypsum wallboard on each side is recognized as having inadequate acoustical performance. That single stud wall has been laboratory tested to an STC 34—well below building code requirements. A similar wall configuration consisting of two layers of type X gypsum wall board on one side and a single layer of type X gypsum board on the other is an STC 36—only a slightly better result. In both cases, the rating of the wall is limited by poor transmission loss at 125, 160 and 2500 Hz. In many cases, the performance is about five to ten decibels lower than it is at other nearby frequencies. For example, at 200 Hz, the wall performs about 12 decibels better than it does at the adjacent measurement frequency, 160 Hz. Similarly, the same assembly performs five decibels better at 3150 Hz than it does at 2500 Hz.
Additionally, some walls are not designed to perform well with specific regard to an STC curve, but rather to mitigate a specific noise source. A good example is that of home theater noise. With the advent of multi-channel sound reproduction systems, and separate low frequency speakers (termed ‘subwoofers’) the noise is particularly troublesome below 100 Hz. The STC curve does not assess walls or other partitions in this frequency range. Materials or wall assemblies should be selected to isolate this low frequency sound.
Various construction techniques and products have emerged to address the problem of noise control, but few are well suited to target these specific problem frequencies. Currently available choices include: additional gypsum drywall layers; the addition of resilient channels plus additional isolated drywall panels and the addition of mass-loaded vinyl barriers plus additional drywall panels; or cellulose-based sound board. All of these changes incrementally help reduce the noise transmission, but not to such an extent that identified problem frequencies would be considered fully mitigated (restoring privacy or comfort). Each method broadly addresses the problem with additional mass, isolation, or damping. In other words, each of these is a general approach, not a frequency specific one.
More recently, an alternative building noise control product having laminated structures utilizing a viscoelastic glue has been introduced to the market. Such structures are disclosed and claimed in U.S. Pat. No. 7,181,891 issued Feb. 27, 2007 to the assignee of the present application. This patent is hereby incorporated by reference herein in its entirety. Laminated structures disclosed and claimed in the '891 patent include gypsum board layers and these laminated structures (sometimes called “panels”) eliminate the need for additional materials such as resilient channels, mass loaded vinyl barriers, and additional layers of drywall during initial construction. The resulting structure improves acoustical performance over the prior art panels by ten or more decibels in some cases. However, the described structures are another general frequency approach. In certain of these structures a single viscoelastic adhesive (with damping) is incorporated into the laminated panel. As will be demonstrated later, such adhesive is designed to damp sound energy within a single frequency band with poorer performance in other sound frequency ranges. For this reason, these structures compromise performance in certain frequency ranges in an attempt to best match the STC curve.
Accordingly, what is needed is a new material and a new method of construction that allows for the maximum reduction of noise transmission at low frequencies, high frequencies, or both simultaneously. What is needed is a panel tuned for performance at multiple problem frequencies.
A figure of merit for the sound attenuating qualities of a material or method of construction is the material's Sound Transmission Class (STC). The STC number is a rating which is used in the architectural field to rate partitions, doors and windows for their effectiveness in reducing the transmission of sound. The rating assigned to a particular partition design is a result of acoustical testing and represents a best fit type of approach to a set of curves that define the sound transmission class. The test is conducted in such a way as to make measurement of the partition independent of the test environment and gives a number for the partition performance only. The STC measurement method is defined by ASTM E90 “Standard Test Method Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements,” and ASTM E413 “Classification for Sound Insulation,” used to calculate STC ratings from the sound transmission loss data for a given structure. These standards are available on the Internet at http://www.astm.org.
A second figure of merit is loss factor of the panel. Loss factor is a property of a material which is a measure of the amount of damping in the material. The higher the loss factor, the greater the damping. The primary effects of increased panel damping are reduction of vibration at resonance, a more rapid decay of free vibrations, an attenuation of structure-borne waves in the panel; and increased sound isolation.
Loss factor is typically given by the Greek symbol “η”. For simple coating materials, the loss factor may be determined by the ASTM test method E756-04 “Standard Test Method for Measuring Vibration-Damping Properties of Materials.” This standard is available on the Internet at http://www.astm.org. For more complicated structures, such as the ones described in the present invention, a nonstandard test method or computer model must be employed to predict or measure the composite material loss factor. A loss factor of 0.10 is generally considered a minimum value for significant damping. Compared to this value, most commonly used materials, such as wood, steel, ceramic and gypsum, do not have a high level of damping. For example, steel has a loss factor of about 0.001, gypsum wallboard about 0.03, and aluminum a loss factor of about 0.006.
In order to design or assess the damping properties of a laminated panel that uses constrained layer damping, a predictive model is used such as the well known model first suggested by Ross, Kerwin, and Ungar. The Ross, Kerwin, and Ungar (RKU) model uses a fourth order differential equation for a uniform beam with the sandwich construction of the 3-layer laminated system represented as an equivalent complex stiffness.
The RKU model is covered in detail in the article “Damping of plate flexural vibrations by means of viscoelastic laminae” by D. Ross, E. E. Ungar, and E. M. Kerwin—Structural Damping, Section IIASME, 1959, New York, the content of which article is herein incorporated by reference. The topic is also well covered with specific regard to panels by Eric Ungar in Chapter 14, “Damping of Panels” in Noise and Vibration Control edited by Leo Beranek, 1971. An extension of this model to systems with more than three layers has been developed by David Jones in section 8.3 of his book Viscoelastic Vibration Damping. This model is used in all of the predictive calculations used for the present invention.